1. Field of the Invention
The invention relates to a process for making the surface of polymeric substrates permanently hydrophilic with hydrophilic monomers using a macroinitiator as primer. It also relates to products having hydrophilic surfaces made by the process which are suitable for use for technical, medical or biotechnical purposes.
2. Discussion of the Background
Polymers (or plastics) having a hydrophilic surface possess various advantages, depending on the intended use, over polymers with a hydrophobic surface. The greater surface energy of hydrophilic surfaces results in better wettability with water, aqueous solutions or dispersions and with other liquids having a high surface tension. Improved wettability is, for example, useful or indeed necessary if a plastics surface is to be printed or dyed with polar dyes or if plastics surfaces are to be joined together using a polar adhesive. In addition, fibers and textile wovens or nonwovens made from polymers require good wettability for sizing, finishing and dyeing.
Hydrophilic surfaces are also important in the case of polymeric materials which are used in aqueous systems. For instance, technical membranes whose purpose, for example, is that of sea water desalination must be easily wetted in order to fully develop their separating effect. The surfaces of plastic pipes or chemical apparatus must be readily wettable where good heat exchange with the environment or, in the case of pipes, where good flow is critical. Good wettabilty is also advantageous for beds of polymer particles through which flow takes place, for example ion exchange resins, and for similarly flow-traversed beds of porous layers, for example dialysis membranes. Another unwanted phenomenon, since it inhibits function, is that of gas bubbles, which settle on the liquid-side surfaces of plastic pipes, hoses or containers because the surfaces are not sufficiently wetted by the liquid.
Plastics with hydrophilic surfaces are indispensable for numerous medical or biotechnical applications because, in contrast to plastics having a hydrophobic surface, they are very highly compatible with blood, tissue fluids or other liquids containing sensitive biological ingredients. Examples of such applications are blood plasma containers, dialysis hoses, catheters, contact lenses, etc.
Polymeric substrates may be made hydrophilic by single-stage or multistage physical and/or chemical treatment. All known treatment processes are aimed at creating hydrophilic groups, such as hydroxyl, carboxyl or keto groups, on the surface of the polymeric substrate. This can be achieved by processes in which the hydrophilic groups form from peripheral or near-surface layers of the polymer itself. Alternatively, or additionally, it is possible to apply layers of hydrophilic compounds to the surface, with or without pretreatment, and--where vinyl monomers are concerned--to carry out polymerization
The single-stage treatment processes which bring about the desired hydrophilic groups from the polymer itself include flaming techniques (D. Briggs et al., J. Mater Sci. 14, 1979, 1344) and corona treatments (J. M. Lane et al., Progress in Organic Coatings, 21, 1993, 269-284). The hydrophilic surface produced in this way, however, is frequently unstable and degrades within hours or days. Plasma processes have also been disclosed, which produce the hydrophilic groups in one stage from the polymer itself. According to W. Mohl, Kunststoffe 81 (1981), 7, polyethylene or polypropylene is treated with low-pressure plasma and is then more suitable for the production of composite materials. Similarly, J. F. Friedrich et al. in GAK 6/94, Volume 47, 382-388 describe a plasma pretreatment of polymers, for example polyolefins, by means of which they can be better bonded with polyurethanes. Plasma processes give satisfactory results if the substrates are bonded soon after the treatment. The hydrophilic properties can be stabilized by further reaction, for example, with hydrophilic monomers. By the above means, chemically bonded, hydrophilic and possibly bulky groups are produced on the surface which are unable to migrate into the interior. However, plasma processes frequently lead to instances of erosion, which make the surface rough. This is frequently unwanted, for example, if the purpose of making the surface hydrophilic is to reduce the frictional coefficient with water. Roughening the surface impairs the tribological properties and acts contrary to this goal.
As a result of a single-stage oxidative treatment with chromium(VI) acid, hydrophilic groups form on the surface of polypropylene from the layers close to the surface (Kang-Wook Lee et al. in Macromolecules 1988, 21, 309-313). However, chromium(VI) compounds are to be avoided where possible in industry because they are carcinogenic and are not permitted to enter the environment.
In other known processes the hydrophilic groups are introduced by coating the substrate with a hydrophilic coating material. A distinction can be made here between processes with and without pretreatment of the polymeric substrate surface, for example, by means of laser, plasma, etc. (the initial cleaning of the surface with a solvent, which is envisaged for almost all of the relevant processes, is not regarded as pretreatment).
One coating process without pretreatment of the substrate is the grafting of polypropylene with 2-hydroxyethyl methacrylate (HEMA), which has been described by S. R. Shukla et al. in J. Appl. Polym. Sci., Vol. 51, 1567-74 (1994). If polymerization is initiated with UV radiation in this case the additional use of methanol as a solvent is necessary, which is toxicologically unacceptable and pollutes the waste water. If the polymerization is initiated by means of uranyl nitrate or cerium ammonium nitrate, it is necessary to prevent the passage of the heavy metals uranium and cerium into the waste water.
The coating processes without pretreatment of the substrate also include the procedure of B. D. Ratner et al., U.S. Pat. No. 5,002,794, in which hydrophilic substances such as polyethylene glycol or 2-hydroxyethyl methacrylate (HEMA) are deposited by means of plasma on metallic, silicatic or plastics surfaces. Hydrophilic monomers, such as HEMA, in this case polymerize spontaneously under the influence of free radicals which are produced by the plasma. H. Mirzadeh et al., Biomaterials, 1995 Vol. 4, No. 8, 641-648 mention the grafting of acrylamide or HEMA onto a specific polymer, namely vulcanized ethylene-propylene rubber. with the aid of a pulsed CO.sub.2 laser. According to S. Edge et al., Polymer Bulletin 27 (1992), 441-445, poly(etherimides) are made hydrophilic without pretreatment of the surface by photochemical grafting of HEMA from the vapor phase. The radiation source used for this purpose is a mercury vapor lamp. Furthermore, according to B. Jansen et al., J. Polymer. Sci., Polymer Symposium 66 (1979), 465-473, a specific polyurethane, Tuftan 410 from B.F. Goodrich, can be grafted with HEMA by irradiation with gamma rays from .sup.60 cobalt. The disadvantages of this process are the expensive radiation protection measures required.
It remains an open question whether, in the case of the processes mentioned in the preceding paragraph, the radiation or the plasma brings about only the polymerization of the monomers or simultaneously activates the surface of the polymeric substrate as well. The latter is presumably the case, since on the one hand, as mentioned above, the effect of the plasma and of corona treatments in making plastics surfaces hydrophilic is known. In any case, both the radiation and the plasma are of such high energy that the hydrophilic monomers, and the polymer formed from them, are attacked. H. Yasuda refers accordingly in J. Polym. Sci.: Macromolecular Review, Vol. 16, 199-293 (1981) to the undefined and uncontrollable chemistry of plasma polymerization. The fact that molecules are destroyed in the course of this process can be demonstrated, in the case of surface coating with HEMA, by the fact that ESCA (Electronic Spectroscopy for Chemical Analysis) in accordance with H. Morra et al., J. Biomed. Mat. Res., 29, 39-45 1995 gives lower than expected values for oxygen in plasma-polymerized HEMA than those which are actually found for HEMA polymerized in a conventional manner, i.e. by a free-radical method. For some applications this may be unimportant. However, in the case of medical or biotechnical applications, a layer of intact HEMA is highly desirable since, as already mentioned, such layers are very highly compatible with the sensitive ingredients of the liquids involved in these applications.
Processes have also been disclosed in which coating with polymerizable monomers is preceded by an activating radiation treatment which modifies the surface of the plastic. Activation and coating of the surface are therefore carried out at separate times. P. Gatenholm et al., Polym. Mater. Sci., 1992, 66, 445-6 describe making polypropylene films and microporous membranes hydrophilic by treatment with ozone and subsequent coatings with HEMA, the polymerization of which is initiated by dissociation of the hydroperoxide groups formed on the surface. A disadvantage of this process is that a relatively high concentration of ozone destroys the polymer. Finally, H. Thelen et al., in Fresenius, J. Anal. Chem. 1995, 353: 290-296, describe a hydrophilic treatment of polyether sulfones in which the substrate is first treated with nitrogen plasma in the presence of small amounts of oxygen and then is coated with HEMA. The process is laborious because the polyether sulfone membrane must be extracted before coating and, as is also the case in the process of Gatenholm et al., oxygen, which inhibits the polymerization, must be carefully excluded from the HEMA solution. Furthermore, the concentration of hydroperoxide groups on the surface, and therefore the grafting density, are difficult to control in the two processes mentioned above.
The literature, moreover, describes peroxide-containing polymers as macroinitiators. In these systems the photolytically or thermally cleavable groups are located in the polymer framework (DE 30 44 531, EP O 370 477). When these groups are cleaved with the formation of free radicals, the polymer framework is broken down and thus loses its stability. A polymer of this kind cannot be applied to other substrates by simple dipping, with formation of a permanent interpenetrating polymer network (IPN), since the cleavage of the abovementioned groups leads to formation of small polymer pieces, which diffuse out of the substrate network.
DE-A 22 42 818 mentions the preparation of polymers whose side chains contain peroxydiester groups but for which no specific application is mentioned.